Combining rg58u with rg59 u coax

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KX2P, Jan 6, 2019.

1. KX2PHam MemberQRZ Page

How much loss can I expect for a 20 meter dipole fed with about 50 feet of coax that is half 50 ohms (RG58) and half 75 ohms (RG59)? Would the loss in db be significant?

2. VE3EKJHam MemberQRZ Page

It's not that simple. The two cables have different impedances, one being 52 ohms and the other being about 75. So the mismatch will make the SWR rather screwy, since the RG59 section will actually act as an impedance transformer, changing the 50 ohms to something a PITA to predict with a random length of RG59.

That being said, it may not be beyond the range of a rig with an Automatic Tuner. As for losses, RG59 has a bit better loss figure for 20 metres but overall the difference at that frequency
is mice nuts.

If you were to make the RG59 length an electrical half wave long for 20 metres it would repeat what impedance it sees at both the beginning and the end. In effect, it would look like 50 ohms if it followed the RG 58. If you don't know how to calculate an electrical half wave of a coaxial cable do a google - there are lots of calculators on line that will help. Or you could google up the formula yourself - the math is fairly simple. You won't even need a calculator - just a pencil and the back of an envelope.

Bill VE3EKJ

3. AI3VHam MemberQRZ Page

Assuming a 50 ohm non reactive antenna, with the 50 ohm coax connected to the antenna, adding a section of 75 ohm coax between the 50 ohm coax and transmitter will result in a 1.5:1 vswr at the transmitter.

This will result in about .02 db additional loss due to vswr on the 75 ohm line on 20 meters with 25 feet of rg59

Assuming 3db per s unit:

3/.02= about 1/150 of a s unit.

I would recommend a transmitter that has a tunable output impedance. Either a built in or external tuner on a solid state rig, a tube rig will tune up just fine.

Rege

Edit: in the real world, a dipole is rarely 50 ohms, transmitters rarely output exactly 50ohms, and vswr meters cannot tell the difference between a 1.2:1 and a 1.5:1 vswr.

The math is much more theoretical than practical.

Edit: the rg59 has about .06 db less matched loss than the rg 58, so you will actually be about 1/50 of a s unit stronger .

Mice nuts for sure.

Last edited: Jan 7, 2019
K7TRF likes this.
4. W4KJGSubscriberQRZ Page

There is so much free, or extremely low cost, 75 Ohm coax available, I would suggest not even fooling with the RG-58.

The center of a balanced half-wavelength long dipole is around 75 Ohms. Your transmitter isn't going to really care if it is not exactly 50 Ohms. It will be quite happy with 72-75 Ohms, including a moderate amount of reactance.

Going to a place like a Habitat for Humanity's ReStore store (https://www.habitat.org/restores/shop) usually will result in more RG-6 and RG-59 that you can carry out for just a few dollars. Weather-proof connectors, including BNC connectors, are readily available from big-box home improvement stores, if the coax doesn't already have connectors on it.

I've switched nearly all of my antenna cabling over to RG-6 and RG-59. Even my ugly baluns are made with RG-6.

Ken

KB0TKZ likes this.
5. W5DXPHam MemberQRZ Page

There's a way to convert Z0s without suffering reflections but I can't remember the lengths - it's something like this.

--------50 ohm coax-------+---75 ohm coax---+---50 ohm coax---+-------75 ohm coax----------75 ohm load

I just can't remember the two middle lengths --- 0.____ wavelength? Anyone remember?

6. W1VTHam MemberQRZ Page

I wouldn't worry about it. Sure you can create transmission line transformers with specific lengths of coax of different impedances. But in your application it won't be a big deal.

Here's a model of a 20m dipole at 35 feet fed with 50 feet of RG-58:

Here's the same antenna fed with 25 feet of RG-58 coupled to 25 feet of RG-59:

In either case your match across the band is very good and your coax losses will be very low.

WB5YUZ likes this.
8. KC8VWMHam MemberQRZ Page

It should exhibit less loss than a 50 foot length of wireless coax.

Last edited: Jan 7, 2019
WB5YUZ likes this.
9. DJ0AJHam MemberQRZ Page

good morning my friend chuck KC8VWM vy73 DJOAJ ekrem

10. KC8VWMHam MemberQRZ Page

Good morning Ekrem my friend. Hope all is well